The marginal gains industry sold a story. Here is what the data actually supports: sleep matters more than anything you can buy, and the shoe matters less than the shop says.
Sleep
Athletes who averaged under seven hours across a season missed 34% more training days through illness. That is the largest effect in the dataset, and it costs nothing.
Shoes
The best modern racing shoe is worth about 2% over a rival’s from five years ago. The advantage between this season’s model and last season’s is inside the measurement error.
- Carbon plates help on the road; on grass the effect disappears.
- Foam degrades after 300km, faster than any brochure admits.
- The placebo effect of a new shoe is real, measurable, and about a week long.
The one percents
They exist. They are just not for sale.